1. Field
The disclosed concept pertains generally to array systems and, more particularly, to such array systems including a plurality of direct current electrical generating modules, such as, for example, photovoltaic electrical generating modules. The disclosed concept also pertains to multiple arrays such as string arrays, including a number of strings having a plurality of direct current electrical generating modules. The disclosed concept further pertains to direct current electrical generating modules and, more particularly, to photovoltaic (PV) modules and detectors and protectors therefor.
2. Background Information
It is known to employ several parallel strings of series-connected direct current (DC) electrical generating modules (e.g., without limitation, PV generating modules) with a protective device located in the positive conductor of each string. This protective device is a fuse and only protects against a reverse over current when the corresponding string shorts and is back fed by the other PV strings which are bussed together at a main DC bus in a combiner box.
In PV generating modules, bypass diodes across cells limit fault voltage, typically to about 11 VDC (for 18 cells) for a polysilicon technology, thereby preventing sustained series arcing internal to the module. However, an open failure of any bypass diode can produce a series arc itself (as it opens), as well as remove the 11 VDC limitation across the cells in parallel with the diode.
PV generating modules themselves typically can achieve voltages from about 35% to about 70% of open circuit voltage, generally in a range that can drive a continuous series arc (in fact, recent developments describe modules with voltages that reach 120 VDC). Thus, a butt joint or an interconnect break within a module shorted at its terminals can produce an internal arc with maximum available power approaching rated power.
Parallel shorts and arcs can have varying available voltages from relatively very low to bus level, depending on their location. A fuse-protected string (typically with reverse current protection of about 8 A) can remain intact (un-blown) for a parallel pinch fault of about two-times rated current at a voltage on the order of about one-third to one-half of the bus voltage (feed). Half the current is sourced from the remote modules, and the other half is sourced from the feed modules. An improvement is a blocking diode that prevents any reverse current, although reliability is a concern here. However, modules which can be switched open through local detection of a string fault could supplement the function of a blocking diode or fuse.
An intermediate case is that of a junction box terminal short or arc. Here the voltage is limited to the module voltage, and the fault current can be viewed as being the difference between the string current and the module current, which is often less than rated current, depending on the fault voltage. On the other hand, if this type of fault is contained within the box itself, local heating (watts) can be higher.
Another type of fault is arcing or leakage to the module frame or other grounded metal. A fuse may exist from the ground system to the neutral (typically about 1 A), which may limit the ground fault current. However, if the fault can be cleared before the fuse blows, the fuse could remain intact as a backup for future incidents.
There is room for improvement in strings.
There is also room for improvement in protection, monitoring or indication apparatus for a direct current electrical generating apparatus.
There is further room for improvement in protection apparatus for a plurality of strings.